1. Field of the Invention
The present invention relates to an electronic circuit for supplying current to an electrical device from a source of alternating or direct current with different voltage levels.
2. Description of the Prior Art
Electronic devices are known for supplying constant voltage and/or constant current to electrical or electronic appliances which are built as choke inverters, primarily or secondarily pulsed blocking or conducting oscillators or as push-pull oscillators. As a rule, they include a rectifier circuit with a filtering and smoothing arrangement connected downstream, to which a transformer or choke is connected. In the case of a choke inverter, an electronic switch or switching transistor is provided in series with the choke and the electrical device, the switch or transistor being switched on and off as a function of one or more control parameters, whereby the energy stored in the choke in the blocked phase of the electronic switch is delivered to the electrical device via a diode with the correct polarity.
A literature reference, "Transistor-Handbuch" (Transistor Handbook) by Jan-Hendrik Jansen (Franzis-Verlag, 1980, pp. 347-354), teaches an electronic device with a choke inverter, wherein an input alternating voltage is applied to a power rectifier, the voltage then being filtered and smoothed before being applied to a switching transistor. A choke is connected downstream of the switching transistor, to which choke a load with a capacitor connected in parallel is connected on the output side. A diode is connected to the load or capacitor on the one hand and on the cathode side to the connection of the switching transistor with the choke. At the load, a measuring circuit measures the amount by which the output voltage determined by the switching times of the switching transistor differs from a reference voltage supplied by the measuring circuit. Variations are fed to a pulse generator which adjusts the width of the control pulses accordingly. The pulses are transmitted to the switching transistor via a control circuit.
To operate an electrical appliance such as an electrical dry shaver, an electronic flash unit, a radio or television set or the like, independently of line voltage, the electrical appliances include a rechargeable battery, for which a regulating circuit must be provided to charge the battery with a constant current, the regulating circuit ensuring a constant output current for charging the battery even with fluctuations in the input voltage varying from 90 to 240 volts, for example. This is particularly necessary when so-called nickel-cadmium batteries are involved, whose charging current may vary only within a very narrow range, since otherwise there would be the danger of the expensive nickel-cadmium cells being destroyed. It is also desirable to connect the electrical appliance in question to different line voltages and frequencies, without having to change the switch setting, something which is frequently foregotten anyway, thereby endangering the electrical appliance in question.